1. Field of the Invention
The present invention relates to an optical disc device which is used when reproduction of information recorded on an optical disc is performed, or recording of information on an optical disc is performed.
2. Description of Related Art
Recently, capacity of information that can be recorded in an optical disc has been greatly increased, and a blu-ray disc (hereinafter referred to as “BD”) has become gradually a mainstream of the optical disc. In the optical disc device utilized for reproducing information recorded on the BD, or for recording information on the BD, in order to make size of a light spot formed on the optical disc small, wavelength (λ) of light from a light source is made short, and numerical aperture (NA) of an object lens is made large (for example, λ=405 nm, NA=0.85).
In general, it is well known that if thickness of a transparent cover layer which protects information recording layer of the optical disc varies, spherical aberration is caused, and amount of generation of the spherical aberration is roughly in proportion to biquadrate of the numerical aperture of the object lens and in inverse proportion to the wavelength. By this reason, the optical disc device corresponding to BD is easily affected by the spherical aberration in comparison with the optical disc device corresponding to CD or DVD, a spherical aberration correcting member to correct the spherical aberration is necessary to be provided.
From past, various kind of the spherical aberration correcting member has been proposed. Among them, there is one in that a collimator lens is controlled and driven, or one in that a beam expander is controlled and driven (See, for example, Patent Document 1 or 2). In those, the spherical aberration correction is performed by that a position along a direction of light axis of a movable lens such as the collimator lens or the like is adjusted by a lens moving mechanism, and a converging state or a diverging state of the light entering an objective lens is made change.
Among the optical disc devices that are provided with such kind of the spherical aberration correcting member, there are some devices that employ structure in that, for example, when reproduction or recording (hereinafter, there may be a case where it is merely referred to as “reproducing or the like”) of the optical disc is begun, first, position of a movable lens is changed, then, servo control is begun to perform reproducing or the like based on positional information of the movable lens which is beforehand recorded in memory.    [Patent Document 1] JP-A-2006-134498    [Patent Document 2] JP-A-2007-265578